1. Field of the Invention
The invention relates to a portable radio apparatus. In particular, this invention relates to a structure of a slot antenna member in a casing.
2. Description of the Related Art
Conventional portable radio apparatuses employ a ferrite antenna, a small loop antenna, a plate-shaped loop antenna or the like. The reception efficiency of such an antenna is determined by a ratio of a wavelength of a used electric wave to an antenna length.
Thus, a portable radio apparatus which employs a loop antenna must be used at high frequencies. To allow the portable radio apparatus usable in, for example, a VHF bandwidth, the aperture area of the loop antenna must be increased, thus making a reduction in the size of the portable radio apparatus difficult. Where .eta. is the antenna efficiency, .gamma.rad is the radiation resistance and .gamma.loss is the antenna resistance, antenna efficiency .theta. is expressed by the following equation: EQU .eta.=.gamma.rad/(.gamma.rad+.gamma.loss) (1)
Radiation resistance yrad is proportional to the square of the aperture area of the loop antenna. Antenna resistance .gamma.loss is proportional to the antenna length, and is inversely proportional to the surface area of an antenna member. Thus, to achieve a reduction in the aperture area of an antenna member and an increase in the antenna efficiency .eta. at the same time, the surface area of the antenna member must be increased, that is, restrictions are imposed on the shape of an antenna member, resulting in an increase in the width of the antenna member. If a wide loop antenna is accommodated in a casing having a curvedly bulging side surface, useless space is generated within the casing.
An antenna having a wide surface area is described in JP 1-34414. The surface of the loop antenna parallel to the aperture surface forms a vertical thick surface. Thus, it is apparent that accommodation of such a loop antenna in the above-described casing generates useless space therein.
In small portable radio apparatuses, a circuit board must be disposed near an antenna because the space in the casing is limited. In such a layout, the loop antenna is influenced by an electronic circuit on the circuit board, deteriorating the sensitivity thereof. Particularly, if a direct conversion type radio apparatus circuit is employed and since the frequency of a locally oscillated signal is almost equal to the reception frequency, the locally oscillated signal and the noise occurring in the locally oscillated signal interfere with radio transmission and reception between that radio apparatus and other radio apparatus. In order to eliminate such a problem in a small portable radio apparatus which employs a loop antenna, the antenna must be disposed at a position separated from the local oscillation circuit, or an effective shielding structure must be provided to suppress an electromagnetic radiation from the local oscillation circuit. Thus, the use of a loop antenna precludes a reduction in the size of the portable radio apparatus.